The present invention relates to furnace wall soot blowers for cleaning ash deposits from the walls of the furnace of a fossil fuel fired steam generator and, more particularly, to a soot blower system for selectively operating individual soot blowers on an independent basis in response to the buildup of ash deposition on the furnace wall in the vicinity of each soot blower.
In steam generators wherein an ash bearing fossil fuel, such as coal, lignite or refuse, is burned, there has always been a problem associated with the deposition of ash formed in the combustion process and carried by the hot combustion products to the walls of the furnace. These ash deposits effectively act as insulation on the walls of the furnace thereby reducing heat transfer from the hot combustion products to the furnace walls which are typically formed of a series of laterally adjacent water cooled tubes welded together to form a gas type enclosure defining therein the combustion chamber of the furnace. As the water flows through these tubes it is heated by radiative heat transfer from the hot combustion products within the furnace to the tube walls of the furnace through which the water flows to generate steam.
As the amount of ash deposition on the furnace walls increases, the heat transfer to the furnace walls steadily decreases. Thus, when the furnaces are very clean as in the stages of initial operation, the heat transfer from the hot combustion products to the furnace wall tubes is very high and the temperature of the combustion products leaving the furnace is at a relatively low value. However, as the furnace walls become dirty from ash deposition, the heat transfer from the hot combustion products to the furnace wall tubes is significantly reduced and the temperature of the hot combustion products leaving the furnace significantly increased. This change in the heat balance over a period of operation of the furnace can cause significant problems for the operator in balancing steam generation. Therefore, it has become customary on furnaces firing ash bearing fossil fuels to install a plurality of soot blowers at various locations in the walls of the furnace over the heighth of the combustion chamber to intermittently clean the furnace walls. The soot blowers are well known in the art and typically involve spraying a blowing medium such as compressed air, water or steam from a spray nozzle head which is intermittently passed through an opening in the furnace wall into the furnace to direct the cleaning fluid under pressure against the surface of the ash deposit. The blowing medium causes thermal shock and high impact loading on the ash deposit thus causing the ash deposit to fall from the furnace wall thereby resulting in a relatively clean furnace tube again being exposed to the hot combustion products.
The deposition of ash on the furnace walls is not uniform over the height of the furnace walls or even over the width of the furnace walls. Certain areas of the furnace tend to receive rapid high ash deposition while other areas of the furnace receive very low ash deposition and remain relatively clean. It is extremely difficult, if not impossible, to predict the exact ash deposition profile which will occur in any given furnace firing any given fossil fuel. Thus, it has become customary to provide a control system for operating the soot blowers of the furnace in an automatic mode, usually according to a preselected time sequence. Each of the individual soot blowers would be operated, typically a row at a time, at set time intervals based upon operating experience. Such a control system is not always entirely satisfactory as relatively clean areas of the furnace may be blown too frequently causing excessive tube wear and unnecessary and expensive use of soot blowing medium while dirty areas of the furnace may be blown too infrequently thereby never achieving a relatively clean furnace condition in those areas. Thus, there is a need for a soot blower system wherein the furnace wall surrounding each soot blower is cleaned selectively as needed.
One scheme for operating soot blowers on a selective basis is disclosed in U.S. Pat. No. 3,276,437. As disclosed therein, each soot blower is operated selectively to clean the furnace wall associated with each soot blower in response to the local furnace wall temperature. Thermocouples are welded to the furnace wall tubes in the vicinity of each soot blower to sense the actual surface temperature of the furnace wall. These wall temperatures are then compared to a set point temperature calculated to be representative of a dirty furnace condition at the particular saturation temperature of the water flowing through the water cooled tubes of the furnace wall. When the sensed temperatures in any one zone fall below this set point temperature, the soot blower associated with that zone is activated. In this manner, the various soot blowers are activated to clean the zones of the furnace with which they are associated in response to furnace dirtiness.
However, furnace wall temperature is not always an adequate measurement of furnace dirtiness. The wall temperature at any particular point on the furnace wall depends on the saturation temperature of the fluid flowing through the water wall tubes. Unfortunately, the local fluid saturation temperature varies with elevation and also with the presence of subcooling at the water wall fluid entrance. Thus, it is very difficult to obtain a true fluid saturation temperature for calculating the preset temperature indicative of furnace dirtiness. Further, on a supercritical steam generator wherein a mixture of water and steam is passed through the tubes at a pressure above the supercritical point of water, there is no way of calculating or determining the metal temperature which would be indicative of furnace dirtiness as metal temperature will vary significantly over the height of the unit dependent not only on the local heat flux but also on the phase state of the mixture flowing through the tubes at that location which is an unknown. Therefore, the control system disclosed in U.S. Pat. No. 3,276,437 would not perform satisfactorily on a furnace of a supercritical steam generator.
It is therefore, an object of the present invention to provide a soot blower system for selectively cleaning the tube walls of the furnace in response to the local heat transfer rate and not local wall temperature.
It is a further object to provide a means for indicating the need for soot blowing in the general area of the furnace wall surrounding a particular soot blower.